Most commonly used in present are multiple-deck screening machines wherein the separation of polydisperse materials according to size is accomplished by letting the material pass through a number of screens placed one beneath another. In this case, to provide an efficient separation of the material on each of the screens, vibrations with specific parameters should be assigned to each individual screen, and the observance of the parameters will depend on the permanent contact between the pushers transmitting these vibrations from the vibrating drives to the screens.
Known in the art is a vibratory screening machine (VEB Chemieanlagenbau Stassfurt Kombinat, DDR--"Vibration technology" Vibratory Screening Machines with Pushers) comprising a frame, a pair of screens prestressed (pretensioned) and mounted in the frame one beneath the other, electromagnetic vibrating drives, and pushers connected to the vibrating drives and extending through both of the screens. Attachment of the pushers to each screen is made by means of spring washers with elastomeric gaskets interposed between them. Thus vibrations with parameters corresponding to those of the pushers are imparted to both the screens.
One disadvantage of the above device is that such attachment of the pushers to the screens requires holes to be made in the screens, with the consequent loss of integrity of the meshes. On the one hand, it results in a reduced service life of the screen plate, and on the other hand, during operation of the machine, in the course of time, it leads to the screen sag in the inter-pusher area, due to the slackening of the pretensioned screens. It is practically impossible to avoid this screen sag. In the areas mentioned, the vibrational effect is insufficient to provide an effective screening of the material, resulting in its accumulation, whereby the overall screening effect is impaired. The provision of the required pretensioning of the screens involves a labour-consuming process of rather frequent screen replacements.
Known is a vibratory screening machine (SU, A, 1189508) comprising a frame, a pair of screens mounted within the frame one beneath the other, electromagnetic drives, and two sets of pushers, the pushers of the first set being connected with the vibrating drives and contacting the screen closest to the drives, the pushers of the second set being each disposed in the interscreen space in alignment with the pushers of the first set, inside a guide rigidly secured with respect to the frame. These guides are formed by hollow cylinders, and the pushers of the second set fitted loosely therein and contacting both the screens are sphere-shaped bodies.
During operation of the screening machine, the pushers of the second set are displaced towards the pushers of the first set by the action of elastic forces exerted by the screen farthest removed from the vibrating drives. Therefore, this screen is a means for displacing the pushers of the second set towards the pushers of the first set.
In the above device, the necessary screen elasticity, in operation, is maintained by a tensioning means. Nonuniform elasticity of the screen over its surface area will cause the steady contact between part of the pushers of the first and second sets to be lost as a result of insufficient travel speed of the second set of pushers, thereby leading to a less reliable vibration transmission from one screen to another It results in a nonuniform screening of the material over the surface of the screens, and consequently in deterioration of the overall screening effect.